When you say calculate current rating of the motor,it means current drawn by the motor from the mains is to be calculated.
For this you should know first what is the output rating in HP or KW.
From this known value of mechanical power output,you can know the electrical power input only by knowing the efficiency of conversion of electrical energy to mechanical energy.
From the known values of Mechanical power output and efficiency you can calculate the Input electrical energy in KWH.The line voltage for star connection is 1.732 times the phase value.If Power factor value is given,it is known or assumed as 0.8.
Now the formula,which you know, KWH = Line Current * Line Voltage * Power factor can be applied.
The only unknown value ‘current’ can be calculated.